1. Field of the Invention
The present invention relates to an image forming apparatus forming an image by using a belt member.
2. Description of the Related Art
An image forming apparatus of this type is disclosed in Japanese Patent Application Laid-open No. Hei 5-232764. This belt member is a photosensitive belt having light transmittancy. This photosensitive belt has no seam, is supported by a drive roll and a tension roll, and runs with the rotation of the drive roll. Around the photosensitive belt, toner developing units are disposed. An electrostatic latent image formed on a front surface of the photosensitive belt is developed with a toner to become a toner image.
Further, Japanese Patent Application Laid-open No. 2005-321552 discloses an apparatus developing a color image on a photosensitive belt. In this apparatus, four developing units for color development are used. The developing units for the respective colors are arranged so as to be capable of coming into contact with the photosensitive belt, and latent images formed on a front surface of the photosensitive belt are developed with color toners. These toner images are sequentially overlaid one on another on an intermediate transfer belt, and one composite color image is transferred to a paper.
Here, each of the aforesaid publications describes that the toner adheres to a rear surface of the photosensitive belt to contaminate the photosensitive belt as the toner image is formed on the photosensitive belt. In each of the publications, a cleaning member is disposed on the rear surface side of the photosensitive belt, and this cleaning member scrapes off the toner on the rear surface of the photosensitive belt as the photosensitive belt runs, thereby removing contaminants on the photosensitive belt.
The contaminants on the photosensitive belt pointed out in Japanese. Patent Application Laid-open No. Hei 5-232764 impairs the light transmittancy of the photosensitive belt to cause poor exposure, which becomes a cause of deteriorating image quality. The contaminants on the photosensitive belt pointed out in Japanese Patent Application Laid-open No. 2005-321552 causes poor grounding of the photosensitive belt and thus becomes a cause of the occurrence of an abnormal image due to insufficient destaticization. Therefore, in both of these publications, it is necessary to remove the contaminants in order to maintain image quality.
In other words, in a case where contaminants on a belt member do not become a cause of deteriorating image quality, a cleaning member is not necessary. For example, in Japanese Patent Application Laid-open No. 2005-321552, the cleaning member, though provided on the photosensitive belt, is not provided on the intermediate transfer belt. This is because, in the intermediate transfer belt in the latter publication, the adhesion of a certain amount of the toner to a rear surface of its transfer surface is not thought to become a cause of deterioration in image quality, provided that the transfer surface has been cleaned.
However, in executing, for example, a transfer process following an image forming process, the toner adhering to the rear surface of this transfer surface gives rise to a problem in the execution of the transfer process in a case where an optical sensor is used to detect the position of the intermediate transfer belt. Specifically, the adhering toner, if scatters, contaminates a light emitting surface and a light receiving surface of the sensor to cause a failure in the detection of the position of the intermediate transfer belt. This does not necessarily become a cause of deterioration in image quality, but is sure to give rise to a problem in the execution of the transfer process.